AM EXAMINERS’ REPORT MAY 2008
AM Physics
May 2008 Session
Examiners’ Report
Part 1: Statistical Information
GRADE
Number
% of Total
A
26
4.60
B
81
14.34
C
135
23.89
D
129
22.83
E
65
11.50
F
97
17.17
Abs
32
5.66
Total
565
100
Part 2: Comments regarding performance
General Comments
Paper 1 – Section A
Question 1
Candidates found this question quite straight forward and most were able to score high marks. It was evident that candidates were able to resolve a vector correctly. Some lost marks in trying to introduce in the picture, formulae for magnetic forces while others were not able to explain the effect of having a smaller weight.
Question 2
Good answers were presented for this question based on the equations of motion for constant acceleration. Students showed less familiarity with the graphs of motion as many failed to draw the proper velocity-time graph. A common error in part (iv) was failure to include the time spent in falling down on the Moon’s surface.
Question 3
Predominantly a question based on forces on a Hooke’s law situation, requiring numerical solutions and a knowledge of simple formulae from statics and dynamics.
Many candidates were not able to provide an explanation and just presented an arithmetic expression and an answer. This habit is to be deplored. Candidates need to realize that the method carries marks.
Question 4
It is surprising that so many candidates still fail to state correctly the Principle of
Moments which, one must stress, is included in the SEC syllabus. The principle does not simply state “clockwise moments = anticlockwise moments” without stating that it is the condition for rotational equilibrium. The other parts of the question were generally well answered.
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AM EXAMINERS’ REPORT MAY 2008
Question 5
Also a well answered question on angular momentum and its conservation. A few candidates were still unable to make a distinction between moment and momentum. One needs to realize that most quantities in Physics carry units which have to be included when presenting final numerical answers. While on the subject of numerical quantities, some still persist in writing down numbers to nine decimal places when the data is obviously much less accurate.
Question 6
The first part of the question, on increasing the conductivity of silicon, elicited fairly good responses in some cases and vague or superficial in others. Candidates were expected to be specific in their answers e.g. in stating valancies and in giving specific examples of donor and acceptor impurity materials. Students were generally able to explain correctly the observations in part (b), based on the correct model for a metallic conductor. A few started from the model from the Band Theory and failed to explain correctly.
Question 7
Common incorrect responses to part (b) of question 7 concerned failure to realize that the emf is numerically equal to the pd when no current flows. Part (c), concerning a sketch graph of the diode characteristics, was not well answered in many cases, especially concerning the breakdown potential and the forward bias features.
Question 8
Generally a well answered question, with weak points on knowledge about the antineutrino and why a positron has a very short half life.
Paper 1 – Section B
Question 9
Good responses to part (a) of this question on linear momentum. Many restricted the definition to collisions failing to realize that the principle applies to interactions including explosions. Very acceptable experimental descriptions were presented. However, few were able to use the principle to explain how a vehicle in space is able to maneuver. In distinguishing between elastic and inelastic collisions, the distinction was expected in terms of conservation of KE and not in terms of whether bodies stick after collision. Part
(c) was well answered except for the necessary assumption.
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AM EXAMINERS’ REPORT MAY 2008
Question 10
Many students who attempted this question presented good responses. It offered candidates no particular difficulties. Regarding the derivations of tangential and radial expressions for the accelerations in part (a), candidates failed to use the easiest approach to do the derivation. The figure was purposely presented sot that v is horizontal at P
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and needs no splitting into components when considering tangential and radial directions.
Question 11
In part (a) marks were lost because students omitted from the stress-strain graph the part corresponding to unloading. They also failed to realize that the area under the graph is proportional to energy per unit volume, or were unable to explain part (v). In part (b), answers expected related to the variable value of the Young’s Modulus and to stiffness/strength. Answers relating to hysterises were discarded as the graph did not show this feature.
Question 12
Those candidates who attempted this question were able to present good answers.
However not many candidates attempted it.
Question 13
Students found this question difficult and correct responses to the numerical parts were rare. Experimental descriptions, however, were good.
Questions 14/15
These two questions were a favourite choice with many candidates. Good answers were given in most cases.
Paper 2 – Section A
Question 1
The students’ performance in this question was of an average quality. The majority of the students mentioned that one can use a thermometer to show that two separate bodies are in thermal equilibrium. However the assumption of having a thermometer with negligible heat capacity was very often omitted or different irrelevant assumptions stated.
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AM EXAMINERS’ REPORT MAY 2008
Question 2
Students’ performance was good in the first parts of this question, however only a limited number of students succeeded in calculating the value of the temperature in part (ci). This was due to the fact that they did not develop a correct initial heat equation. Only a few students mentioned the precaution of stirring the milk during the measurements.
Question 3
The term Electric Potential was often given the same definition of the gravitational potential by using a unit mass instead of a unit charge. As regards to the ‘man entering the iron cage’, many colourful irrelevant explanations were given including ‘man wearing safety shoes’, ‘man discharging the cage before entering in it’ etc. As regards to the charging by induction experiment, very few students mentioned the correct phenomena. However the majority described the experiment quite well. The majority of the students failed to calculate correctly the last parts of the question in which sometimes they used PE as mgh instead of QV.
Question 4
The majority of the students performed well in this question. The main problematic issue that was seen was in part (aii) where students found it difficult to explain the motion of the wire from high flux density to low flux density.
Question 5
The majority of the students performed well in this question. The most encountered difficulty was in part (biv) where a number of students did not explain correctly the differences in the energy stored.
Question 6
The majority of the students gave a correct answer for part (a) of the question. However in the second part which involved mainly the interpretation of the equation to extract data values, a considerable number of students performed poorly.
Question 7
Only a limited number of students showed good performance in this question. In part (ai) reflection and refraction were given as answers for total internal reflection. As regards the conditions for such principle to occur some students drew diagrams of side mirrors along the path of the fibre optic with the convention that light is reflected by the side mirrors.
Only a limited number of students provided a good diagram and explanation of the main components needed for the transmission of a signal through a fibre optic system.
Question 8
The majority of the students performed poorly in this question. They showed lack of knowledge in this subject and their explanations were very often irrelevant namely for viewing red light as an explanation for the red shift. Also students gave an estimate of the age of the universe by stating it incorrectly instead of calculating it.
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AM EXAMINERS’ REPORT MAY 2008
Paper 2 – Section B
Question 9
The majority of the students performed excellently in this question. As regards to part (e), a small number of students provided the right concept but did incorrect mathematical calculations.
Question 10
Students’ performance was good in parts (a) and (b) of the question. The piston with a pressure gauge experiment was the most quoted experiment to demonstrate Boyle’s law.
However it was noted that a considerable number of students provided the wrong graph for the inverse relation between both quantities. The students showed average performance in part (c) of the question. They used incorrect signs for the work done on the system or by the system.
Question 11
The majority of the students performed well in this question. The small sources of error were the units for the Universal Gas Constant and Boltzmann’s Constant and the mathematical calculations in part (c).
Question 12
The majority of the students showed average performance in this question. In part (a ii ) students used the wrong formula while there was evidence of lack of knowledge on the concept of black bodies in part (b). Very few students completed successfully part (c).
Question 13
Students’ good performance was good in this question. They found difficulties in defining the time constant, while in part (c) the incorrect charge-voltage relationship was used. Some students used the formula for the capacitance related to the structure of the capacitor. In part (e) some students mixed up formulae for capacitors in series and in parallel.
Question 14
The students defined clearly both electromagnetic laws but very often failed to apply them in the second part of the question. They found it difficult when solving the directions of the magnetic field perpendicular to the plane of the helicopter‘s blades in part (b). Only a small number of students showed knowledge on the back e.m.f concept in part (c).
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AM EXAMINERS’ REPORT MAY 2008
Question 15
A limited number of students performed well in this question. Students showed lack of knowledge about the term coherence while they used the wrong physical relation to explain part (b v) i.e. to explain the effects on the fringe separation with varying distance between slits.
A-Level Physics Practical Paper
In this year’s practical paper, the candidates were required to investigate the properties of a JFET transistor. The paper was divided into three parts.
In the first part of the paper they were asked to connect a multimeter set to read current in a circuit as well as connect different resistors between two marked crocodile clips. The candidates were then asked to read the current for each resistor placed between the crocodile clips and from the data they obtain, determine whether the component is acting as an Ohmic conductor or not.
In the second part of the paper the candidates were required to measure the drain current and the source voltage on a set of resistors connected in the circuit. From the data they obtain, they were required to plot a graph of these two parameters. An extrapolation of the graph at one end of the curve was then required in order for the students to determine the threshold voltage.
In the last part of the experiment the students were given a relationship between the source voltage, drain current and the threshold voltage and were asked to plot a second linear graph.
The vast majority of candidates succeeded in doing well in this practical. The most common mistakes occurred mostly in the second and third part of the experiment. In the second part of the experiment the most common mistakes that occurred were with the plotting of the graph. Some candidates chose a scale that made it impossible later on, to determine the value of the threshold voltage by extrapolation. As regards the third part of the experiment some candidates chose to adopt the natural log instead of the suggested log to base 10. Although the linear graphs that were plotted were still considered correct the mistakes arose when they had to determine an unknown constant. The constant was in a few cases determined wrongly as the candidates chose the incorrect inverse for the log.
In a few words the logs adopted and their inverse in some cases mismatched. Other common sources of error were in simple calculations of the gradient and incorrect rearranging of the given relationship in the form of (y = mx + c).
On the whole, it seems that most candidates are doing well in their practical session.
They should however pay attention to details mentioned in this report as these are necessary to obtain full marks from this paper.
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AM EXAMINERS’ REPORT MAY 2008
General Remarks
Circuits
It is to be noted that a good number of candidates panic when presented with a practical involving circuits. i) Candidates find it difficult to follow instructions to complete a circuit. ii) It appears that candidates find difficulty in reading voltmeter and ammeter scales and in making the appropriate connections to the instruments. This might be due to the fact that ammeters and voltmeters given during an experiment might not be exactly the same make as those found in their school laboratories. At this level candidates should be able to work out the proper connections and read the scales properly, without any guidance.
Errors and Accuracy
At this level, students should be aware of the type of errors that might creep in when they are performing experiments, but this knowledge will not be asked for during the A-level
Physics examination.
During the Practical session i) Candidates should know how to avoid certain errors while taking readings, eg parallax errors. ii) They will be told the number of repetitions of readings to be taken and when an average is needed. iii) They are expected to read the instruments provided, eg rulers, meters, etc, to the nearest scale reading.
During the theory examination
Candidates can deduce the accuracy expected in the answer from the accuracy of the data provided.
Chairperson
Board of Examiners
July 2008
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